(1) Field of the Invention
The present invention relates generally to phonograph pickup devices having a pivotally mounted tone arm with a cartridge carried thereon and in one aspect to the controls therefor for providing damped movement of the tone arm to a record disc and in another aspect to the controls for establishing a predetermined relationship between the cartridge and the record disc and in particular to such controls using feedback control techniques. In another aspect this invention relates to such phonograph pickup devices wherein both types of controls are used with provision made for selectively controlling the operation of such controls.
(2) Brief Description of the Prior Art
As a result of the light stylus force with which high fidelity record players are operated to minimize distortions and to minimize wear on both the record and stylus, warpage of record discs presents a major problem in that the stylus may tend to skip over grooves in the discs. Such limitations in trackability have historically been partially overcome with the use of viscous damped tone arms. However, such arms have not proven practical, probably due to the elaborate mechanisms involved and due to the long term instability of the damping fluids.
In addition to damping the tone arms, trackability may also be improved by designing the tone arm mass and the cartridge compliance such that the natural period of the combination is just below the audible range. Such an approach has practical limitations.
U.S. Pat. Nos. 3,623,734 (Sakamoto & Goto) and 3,830,505 (Rabinow) disclose two alternative solutions involving the regulation of the relationship between the stylus and record disc to compensate for warpage and thereby to improve trackability. In both of these patents, a deviation signal corresponding to a change in the relationship is used to control a feedback loop to maintain the relationship constant. The system disclosed in U.S. Pat. No. 3,830,505 is designed to maintain a constant distance between the stylus and record disc, while the system disclosed in U.S. Pat. No. 3,623,734 is designed to maintain constant such factors as pressure, distance and velocity. Typical audio velocity sensitive transducers, such as those disclosed in these patents, provide an output signal at low frequencies, e.g., approximately 1/2 Hz corresponding to one revolution at 33 1/2 rpm.